2 tablespoons olive oil or vegetable oil, plus some for drizzling
1/4 pound button mushrooms (5-6), stemmed and very finely chopped
1 carrot, very finely chopped
1 rib celery, very finely chopped
1 shallot, very finely chopped
2-3 cloves garlic, very finely chopped
Salt and pepper
2 large or 3 medium Russet potatoes, peeled and cubed
1 1/2 pounds coarse beef chuck
4 tablespoons Worcestershire sauce, divided
1/4-1/3 cup whole milk
2 tablespoons sour cream
2 tablespoons horseradish
1 egg yolk, beaten
4 tablespoons butter, divided
2 tablespoons chives, finely chopped
2 tablespoons flour
1 can beef consommé
1 tablespoon Dijon mustard
4 slices white bread, toasted and lightly buttered
Chopped flat leaf parsley, for garnish
In a skillet over medium heat, heat 2 tablespoons olive oil or vegetable oil. Add the vegetables and garlic; season with salt and pepper, cook to tender, then cool completely.
Meanwhile, boil the potatoes in salted water until tender. Combine the ground beef with 3 tablespoons Worcestershire sauce, salt, pepper and the cooled vegetables. Form four patties and drizzle them with oil.
Heat a griddle or cast iron skillet over medium-high heat. Cook the burgers for 10 minutes, turning once for medium doneness.
Switch the broiler on high. Drain the potatoes and mash with the milk, sour cream, horseradish, egg yolk, 2 tablespoons butter, chives, salt and pepper. Top the burgers with mounds of potatoes and set them under the broiler to brown, in the middle of oven, 3-4 minutes.
Melt the remaining 2 tablespoons butter in a small skillet over medium heat. Whisk in the flour, then the consommé; let thicken and season with the remaining tablespoon of Worcestershire sauce, black pepper and Dijon. Set the burgers topped with the potatoes on buttered toast and pour gravy over the top. Garnish with parsley and serve.